Exceptionally large crystals of magnetite (Fe3O4) found in Atlantic coastal sediments and dating to 55.6 million years ago may have been formed by eukaryotes during a period of global warming, reports a group of researchers led by Hojatollah Vali of McGill University, in Montreal (Proc. Natl. Acad. Sci. USA, DOI: 10.1073/pnas.0803634105). The researchers found two unprecedented shapes of magnetite crystals: flattened spearheads that are up to 3.8 μm long, 1.4 μm wide, and 0.7 μm high; and cylindrical spindles, tapered on each end, that are up to 3.3 μm long and 0.5 μm wide. The complex shapes are likely to be biogenic in origin rather than formed through normal crystallization, the researchers say. Furthermore, because the crystals are larger than those known to be made by bacteria, they are more likely to have come from eukaryotes, which may have used the particles for their magnetic properties or their hardness. The researchers also identified a shell-like structure assembled from spearheads, with the stems oriented inward and the tips pointing outward, suggesting that the crystals served a structural and perhaps protective purpose around eukaryotic cells.